The present invention relates to a circuit breaker having an overcurrent-tripping device for detecting a short-circuit current to allow an opening-and-closing mechanism to perform a tripping operation, and opening a movable contact shoe before opening the same by the tripping operation of the opening-and-closing mechanism.
FIG. 7 is a vertical sectional view of a conventional circuit breaker of the above design in an ON state. In FIG. 7, a molded case 1 has conduction paths therein, each being formed of a pair of opposed fixed contact shoes 2 and 3, and a movable contact shoe 4 bridging the fixed contact shoes 2 and 3, with a power supply side terminal 5 integrally formed at an end of the fixed contact shoe 2. The movable contact shoe 4 is held in a movable contact shoe holder 6 formed of an insulating material so that the movable contact shoe 4 can move linearly in a vertical direction of FIG. 7, and the movable contact shoe holder 6 is guided in the molded case 1 to enable linear movement. The movable contact shoe 4 is pressed against the fixed contact shoes 2, 3 by a contact spring 7 formed of a compressed coil spring, and inserted between the movable contact shoe 4 and the bottom of the molded case. Thus, the movable contact shoe 4 is contacted with the fixed contact shoes 2, 3 via movable and fixed contacts.
An opening-and-closing mechanism 8 is installed above the fixed contact shoe 2 in FIG. 7, and a tip of an opening-and-closing lever 10 that is rotated around a rotating shaft 9 is located to oppose an upper surface of the movable contact shoe holder 6. When an opening-and-closing handle 11 of the opening-and-closing mechanism 8 is operated in an ON state, the opening-and-closing lever 10 is rotated clockwise in FIG. 7 to depress the movable contact shoe holder 6 against the force of the contact spring 7. The movable contact shoe 4 is then opened from the fixed contact shoes 2, 3 to clear the conduction path.
A thermally operated electromagnetic overcurrent-tripping device 12, which is formed of a bi-metallic element and an electromagnet (not shown), is disposed on the fixed contact shoe 3. The overcurrent-tripping device 12 has one end connected to the fixed contact shoe 3 and the other end connected to a load-side terminal 13 so as to unlock the opening-and-closing mechanism 8 by means of the bi-metallic element, which is bent when an overloading current flows through the conduction path, or by instantaneously attracting the movable iron core of the electromagnet if a heavy current such as a short-circuit current flows through the conduction path, whereby a stored force of an opening-and-closing spring (not shown) rotates the opening-and-closing lever 10 to open the movable contact shoe 4.
In this case, particularly in the presence of a heavy current, the movable iron core drives the movable contact shoe holder 6 downward to open the movable contact shoe 4 before the tripping operation of the opening-and-closing mechanism 8. When the current is shut off to eliminate the attractive force on the movable iron core, the movable contact shoe 4 attempts to rise under the force of the contact spring 7. However, since the rotation of the opening-and-closing lever 10 has been completed, the movable contact shoe 4 is held open. Furthermore, an arc occurs between the movable contact and the fixed contact when a heavy current is interrupted. This arc is guided into arc extinguishing chambers 14 and 15 installed below the fixed contact shoes 2 and 3, where it is extinguished. In order to transfer the current from the movable contact shoe 4, a commutation plate 16 is installed to extend across the arc extinguishing chambers 14, 15.
An example of the above overcurrent-tripping device 12 is described in Japanese Patent Application Laid-Open No. 6-52782. FIG. 8 shows a modified version of this patent. In this figure, the overcurrent-tripping device 12 is formed of a bi-metallic element 17 and an electromagnet 18. The bi-metallic element 17 has a heater 19 wound around it, and the electromagnet 18 comprises a movable iron core 21 inside an electromagnet coil 20. The heater 19 has one end connected to a fixed contact shoe (not shown) and the other end connected to the upper end 17a of the bi-metallic element 17. On the other hand, the bi-metallic element 17 is supported by a bi-metallic element support 22 at its lower end and is electrically connected to the bi-metallic element support 22.
In addition, although not seen in FIG. 8, the bi-metallic element support 22 is connected to a lower end of the electromagnet coil 20, which has an upper end 20a connected to a terminal 23. The electromagnet coil 20 is supported by a coil support 24. The coil support 24 has a lever 25 to enable rotation around a shaft 26, and a tip portion 25a of the lever 25 extends in a direction crossing the bi-metallic element support 22. The other end of the lever 25, which is not shown in FIG. 8, is connected to a lower end of a movable iron core 21. Although not shown, a fixed iron core is joined to a surface of the terminal 23, which is opposed to an upper-end surface of the movable iron core 21, and the movable iron core 21 is forced downward by means of a return spring interposed between the fixed iron core and the movable iron core 21.
A conduction path in this overcurrent-tripping device 12 comprises the heater 19, the bi-metallic element 17, the bi-metallic element support 22, the electromagnet coil 20, and the terminal 23, in that order. When a heavy current such as a short-circuit current flows through this conduction path, the movable iron core 21 is attracted upward toward the fixed iron core, while rotating the lever 25 in the direction of the arrow in the figure. In response to this operation, an opening-and-closing mechanism (not shown) will be tripped to open the movable contact shoe, but before this tripping action, the lever 25 pushes a movable contact shoe holder (not shown) downward to open the movable contact shoe. After the current has been shut off to eliminate any attractive force on the movable iron core, the movable contact shoe is maintained in the opened state by means of the opening-and-closing mechanism, which has performed the tripping operation.
The above conventional overcurrent-tripping device 12 has the following problems:
(1) The movable iron core 21 is attracted to the fixed iron core to rotate the lever 25 in order to push the movable contact shoe holder downward and open the movable contact shoe. Mechanical slack, however, inevitably occurs in a connection between the movable iron core 21 and the lever 25, and in a rotation-support-point portion of the lever 25, so that a time delay occurs between the linear movement of the movable iron core 21 and the rotational movement of the lever 25, resulting in a corresponding increase in operating time. PA1 (2) Since the tip portion 25a of the lever 25 is located adjacent to the bi-metallic element 17, the increase in the thickness of the heater 19 for augmentation of its rating is limited. PA1 (3) Although the lever 25 is used to push the movable contact shoe holder downward to open the movable contact shoe, the movable contact shoe holder's large mass makes it difficult to increase the opening speed of the movable contact shoe by an appropriate degree.
It is thus an object of the present invention to solve these problems in order to improve the shutting-off performance of the circuit breaker while facilitating rating augmentation.